1. Field of the Invention
This invention relates to the field of data modems, and particularly to modem arrangements for efficient use of frequency spectrum allocations in a closely packed transmission system, such as a cable television network.
2. The Prior Art
Most data modems in the past have been constructed for use in telephone systems with the bandwidth limitation normally imposed by television circuits. Other types of transmission media, such as the transmission and reception of radio waves do not have the same inherent limitations of bandwidth as telephone systems but generally lack control of the transmission path that is possible in a telephone system. A cable system arranged to transmit multiple broad band signals, such as television signals, is not inherently limited in bandwidth usage to the same extent as telephone systems but does have good control of the signal path between specific transmission and reception points.
Cable television systems have been installed in high density commercial and residential areas, and are quite suitable for commercial usage such as the transmission of data between branches of a bank or other corporation located at more than one point along a cable system and having a large volume of information that must be carried between the different branches or offices.
It is now recognized as being worthwhile to install a cable communication system entirely for commercial purposes, as opposed to the entertainment purpose originally considered to be the basic reason for cable television networks, in large manufacturing complexes. For example, chemical processing plants require the transmission of so much data for proper management and control that it is justifiable to install a cable television system for that purpose, alone.
Although wide band signals can be transmitted in a cable system, it is still necessary to make efficient use of the spectrum. This means that signals multiplexed together must be restricted to certain limited bands and that the frequency spectrum in these bands must not be wasted. In order to manage the transmission properly, it is desirable that signals be transmitted to a central station at one end of the cable and then retransmitted from that central station. To keep the signals flowing in one direction from interfering with those flowing in the other, the frequencies of incoming signals are shifted at the central station (referred to as the "head end"). This requires careful frequency control, not only to keep signals from one modem separate from another but also to control the signal of each modem so that it can be correctly demodulated.